In a mixing bowl, combine the ground turkey, diced onion, egg whites, grated Parmesan, whole-wheat panko, and minced garlic; mix until evenly combined.
Form the mixture into roughly 1-inch meatballs using your hands.
Heat the olive oil in a large high-sided skillet over medium-high heat until shimmering.
Add the meatballs to the hot skillet and cook 8–10 minutes, turning once or twice so they brown on most sides and cook through; drain any excess liquid from the pan.
Pour the diced tomatoes, marinara sauce, and hot water into the skillet with the meatballs, then add the whole-wheat spaghetti (break noodles in half if needed to fit).
Bring the mixture to a boil, cover, then reduce heat and simmer 10–12 minutes, stirring occasionally, until the pasta is tender; add a splash more water if the pan becomes too dry.
Serve immediately, optionally topping with extra grated Parmesan or chopped parsley if desired.